A typical gas turbine engine comprises an air inlet followed by a compressor section in which incoming air is compressed for application to one or more combustors of a combustor section of the gas turbine engine. A fuel, which may be a gaseous or liquid fuel, is introduced into the combustors and mixed with a part of the compressed air. Hot combustion gas created by combustion of the fuel/air mixture in the combustors is directed to a turbine section comprising a set of turbine blades and a set of turbine guide vanes. The combustion gas flowing against the turbine blades leads to a rotation of a shaft of the gas turbine engine to which the turbine blades are attached. As the blades of the compressor section are also attached to the shaft, a part of the mechanical power generated by the turbine section is used to operate the compressor section.
A combustor of a gas turbine engine usually comprises at least a burner, a swirler, and a combustion prechamber, which is adapted for a downstream fluid communication with a main combustion chamber. A usually planar burner face delimits the prechamber in the upstream direction.
The main purpose of the burner is to introduce fuel and air into the combustion prechamber, whereas a thorough mixing of the fuel and the air is necessary to obtain a stable and efficient combustion with good flame stability and the smallest possible amount of NOx emissions. Therefore, the combustor design must ensure that proper amounts of fuel are introduced in the right locations within the combustor, that these amounts of fuel are thoroughly mixed with air, and that thorough fuel vaporization takes place.
A swirler may be provided to achieve better mixing of fuel and air. A swirler comprises swirler vanes arranged in such a way that compressed air, which is guided through swirler slots being positioned between the swirler vanes, will be forced into a swirling movement around an axial centerline of the combustion prechamber. The swirling movement of the air enhances the mixing of the air with fuel.
In order to decrease the production of nitrogen oxides by gas turbine engines, use is made of so-called lean burn pre-mix combustors, in which the fuel to air ratio is reduced as far as possible in the higher operating ranges. Nevertheless, these lean fuel to air ratios are problematic with respect to maintaining flame stability when the engine load is reduced. It is known to incorporate a pilot fuel system into the burner, which will inject a supplemental amount of (pilot) fuel into the combustion prechamber in order to locally raise the fuel to air ratio.
Usually, the pilot fuel system is also used when starting the combustor. Fuel is injected from the pilot face towards the prechamber and ignited by an ignitor, which may be positioned somewhere within the pilot-burner face.
A combustor which comprises a pilot fuel system is disclosed in U.S. Pat. No. 6,151,899 A1, for example. The pilot fuel system comprises a nozzle for injecting pilot fuel into a combustion prechamber, the nozzle being situated within a central recess of a burner face at such a position and orientation that the fuel is injected substantially tangentially into the recess so as to flow around the peripheral wall thereof. For starting, the fuel is ignited by means of an electric spark ignitor, which may be situated in the pilot burner face.
An alternative positioning for pilot fuel injectors is disclosed in U.S. Pat. No. 6,532,726 B2. With the combustor disclosed therein, a single liquid lance arranged on the burner face.
Liquid pilot fuel is injected by means of the lance in the axial direction of the prechamber. Ignition of the fuel/air mixture is achieved by a spark ignition unit, which is positioned within the burner face as well.
In particular if such a combustor is run on liquid fuel a poor starting reliability may be encountered. This is the case, because ignition requires a sufficient amount of fuel/air mixture with a correct ratio to be present near the ignitor. Achieving this proves to be more difficult with liquid fuel than with gaseous fuel due to the worse mixing of liquid fuel and air compared to gaseous fuel.